Abstract
Mediterranean pine forests display high resilience after extreme climatic events such as severe droughts. However, recent dry spells causing growth decline and triggering forest dieback challenge the capacity of some forests to recover following major disturbances. To describe how resilient the responses of forests to drought can be, we quantified growth dynamics in plantations of two pine species (Scots pine, black pine) located in south-eastern Spain and showing drought-triggered dieback. Radial growth was characterized at inter- (tree-ring width) and intra-annual (xylogenesis) scales in three defoliation levels. It was assumed that the higher defoliation the more negative the impact of drought on tree growth. Tree-ring width chronologies were built and xylogenesis was characterized 3 years after the last severe drought occurred. Annual growth data and the number of tracheids produced in different stages of xylem formation were related to climate data at several time scales. Drought negatively impacted growth of the most defoliated trees in both pine species. In Scots pine, xylem formation started earlier in the non-defoliated than in the most defoliated trees. Defoliated trees presented the shortest duration of the radial-enlargement phase in both species. On average the most defoliated trees formed 60% of the number of mature tracheids formed by the non-defoliated trees in both species. Since radial enlargement is the xylogenesis phase most tightly related to final growth, this explains why the most defoliated trees grew the least due to their altered xylogenesis phases. Our findings indicate a very limited resilience capacity of drought-defoliated Scots and black pines. Moreover, droughts produce legacy effects on xylogenesis of highly defoliated trees which could not recover previous growth rates and are thus more prone to die.
Highlights
Mediterranean forests are able to recover following major disturbances such as droughts by displaying high resilience (e.g., Lloret et al, 2004)
The study area is located in the Sierra de Filabres, Andalusia, SE Spain (37◦ 22′ N, 2◦ 50′ W; see Figure S1). This area was planted with Scots pine (Pinus sylvestris L.) and black pine (Pinus nigra Arn.) in the 1970s, with most stands of each species located at approximate elevations of 1850–2000 m and 1700–1850 m a.s.l., respectively (Sánchez-Salguero et al, 2012b; Herrero et al, 2013)
We found that Basal area increment (BAI) for the 2000−2008 period was significantly lower (P < 0.05) in the case of D trees (Scots pine, mean ± SE = 2.2 ± 0.4 cm2; black pine, 1.6 ± 0.3 cm2) as compared with trees presenting intermediate (I) or low (N) defoliation levels whose mean BAI values did not differ
Summary
Mediterranean forests are able to recover following major disturbances such as droughts by displaying high resilience (e.g., Lloret et al, 2004). Warmer and drier conditions could lead to growth decline of drought-prone Mediterranean conifer forests (Sarris et al, 2007; Sánchez-Salguero et al, 2012a; Galván et al, 2014). Such reductions in productivity may predispose trees to droughtinduced dieback once growth decline and vigor loss become irreversible (Camarero et al, 2015). It is compelling to determine if Mediterranean forest growth recovers after successive or severe droughts and if this response may cause a loss of resilience
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